10.3 Biological Nutrient Removal: Nitrification, Denitrification & Phosphorus Removal

10.3 Biological Nutrient Removal: Nitrification, Denitrification & Phosphorus Removal

[!NOTE] Arizona Aquifer Protection & Class A+ Reclaimed Water Standards: In Arizona, nitrogen management is strictly regulated by the Arizona Department of Environmental Quality (ADEQ) under the Aquifer Protection Permit (APP) program (A.A.C. Title 18, Chapter 9) and Reclaimed Water Quality Standards (A.A.C. Title 18, Chapter 11). Wastewater discharged to groundwater aquifers via vadose zone recharge wells or surface percolation basins must not exceed the Aquifer Water Quality Standard (AWQS) of $10.0\text{ mg/L}$ Total Nitrogen as N (established to prevent methemoglobinemia, or 'blue baby syndrome', in drinking water aquifers). Furthermore, achieving Class A+ Reclaimed Water requires Total Nitrogen to be consistently maintained below $10.0\text{ mg/L}$ as a 24-hour composite average, necessitating advanced Biological Nutrient Removal (BNR).

Nutrient enrichment (excessive nitrogen and phosphorus) in receiving aquatic environments accelerates eutrophication, triggers toxic cyanobacterial blooms, and depletes dissolved oxygen. In water-scarce Arizona, where over 90% of treated municipal wastewater is beneficially reclaimed for indirect potable reuse, groundwater replenishment, agricultural irrigation, or power plant cooling, biological nutrient removal is a fundamental design and operational requirement.

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Biological Nitrogen Removal: Nitrification

Nitrification is a two-step biological oxidation of reduced nitrogen (ammonia/ammonium) to nitrite and subsequently to nitrate. It is carried out by obligate aerobic, chemolithoautotrophic bacteria that utilize inorganic carbon (carbon dioxide, $CO_2$, or bicarbonate, $HCO_3^-$) rather than organic carbon for cellular synthesis.

Two-Step Nitrification Biochemistry

1. Step 1: Ammonia Oxidation to Nitrite: Ammonia-Oxidizing Bacteria (AOB), primarily Nitrosomonas (along with Nitrosococcus and Nitrosospira), oxidize ammonia to nitrite ($NO_2^-$):

$2NH_4^+ + 3O_2 \xrightarrow{\text{Nitrosomonas}} 2NO_2^- + 4H^+ + 2H_2O + \text{Energy}$

1. Step 2: Nitrite Oxidation to Nitrate: Nitrite-Oxidizing Bacteria (NOB), primarily Nitrobacter and Nitrospira, oxidize nitrite to nitrate ($NO_3^-$):

$2NO_2^- + O_2 \xrightarrow{\text{Nitrobacter / Nitrospira}} 2NO_3^- + \text{Energy}$

1. Overall Combined Nitrification Reaction:

$NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O + \text{Synthesized Biomass}$

Critical Stoichiometric Demands (Exam Essentials)

• Stoichiometric Oxygen Requirement:
  • Step 1 requires $3.43\text{ lbs } O_2$ per lb $NH_4^+$-N oxidized to $NO_2^-$.
  • Step 2 requires $1.14\text{ lbs } O_2$ per lb $NO_2^-$-N oxidized to $NO_3^-$.
  • Total Oxygen Demand: $4.57\text{ lbs of } O_2\text{ consumed per lb of } NH_4^+\text{-N oxidized}$.
  • (Note: In a facility achieving complete nitrification, the oxygen demand for ammonia oxidation often accounts for 40% to 50% of the entire plant aeration requirement!)
• Alkalinity Consumption:
  • The oxidation of ammonia releases two hydrogen ions ($2H^+$) per mole of nitrogen oxidized, neutralizing natural bicarbonate buffering:
  • $7.14\text{ lbs of Alkalinity (as } CaCO_3\text{) consumed per lb of } NH_4^+\text{-N oxidized}$.
  • If incoming wastewater lacks sufficient natural alkalinity, nitrification destroys the buffer, causing pH to drop rapidly. Nitrification rates drop steeply below pH 6.8 and cease completely at pH $<6.0$. To sustain nitrification, facilities must add supplemental alkalinity: hydrated lime ($Ca(OH)_2$), soda ash ($Na_2CO_3$), caustic soda ($NaOH$), or magnesium hydroxide ($Mg(OH)_2$).

Environmental Conditions for Optimal Nitrification

• Dissolved Oxygen: Target $>2.0\text{ to } 3.0\text{ mg/L}$. Because autotrophic nitrifiers have a much lower oxygen affinity ($K_{DO} \approx 0.5\text{ mg/L}$) than heterotrophic carbon-oxidizers, low DO ($<1.5\text{ mg/L}$) selectively starves nitrifiers, leading to nitrite accumulation or incomplete ammonia conversion.
• Optimal pH: 7.5 to 8.5. Severe biological inhibition occurs below pH 6.8.
• Temperature Kinetics: The maximum specific growth rate ($\mu_{max}$) of nitrifiers is highly temperature-dependent ($k_T = k_{20} \times 1.072^{T-20}$). Nitrification rates drop by approximately 50% for every $10^\circ\text{C}$ drop in wastewater temperature. In high-altitude northern Arizona winter conditions ($10^\circ\text{C}$), the minimum required aerobic MCRT to retain nitrifiers increases from 5 days (summer) to 15 to 20 days.

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Biological Nitrogen Removal: Denitrification

Biological denitrification is the biological reduction of oxidized nitrogen (nitrate, $NO_3^-$, and nitrite, $NO_2^-$) to inert atmospheric nitrogen gas ($N_2$). It is performed by ubiquitous facultative heterotrophic bacteria (Pseudomonas, Alcaligenes, Paracoccus, Bacillus) in an anoxic environment:

$NO_3^- \rightarrow NO_2^- \rightarrow NO\uparrow \rightarrow N_2O\uparrow \rightarrow N_2\uparrow$

Environmental Operating Requirements

• Strict Anoxic Conditions: Free dissolved oxygen must be $< 0.2\text{ mg/L}$ (ideally $0.0\text{ mg/L}$). In the presence of DO, facultative bacteria preferentially utilize oxygen as their terminal electron acceptor because it yields more metabolic energy, completely repressing the nitrate reductase enzyme system.
• Alkalinity Recovery: Denitrification releases hydroxyl ions ($OH^-$), producing alkalinity:
  • $3.57\text{ lbs of Alkalinity (as } CaCO_3\text{) produced per lb of } NO_3^-\text{-N reduced}$.
  • This restores exactly 50% of the alkalinity destroyed during nitrification, helping stabilize basin pH without supplemental chemical purchase.
• Readily Biodegradable Carbon Requirement: Denitrifiers are heterotrophs that require an organic carbon electron donor. Carbon can be supplied via raw wastewater influent readily biodegradable COD (rbCOD), endogenous cellular decay, or external supplemental carbon:
  • Methanol ($CH_3OH$): $6NO_3^- + 5CH_3OH \rightarrow 3N_2\uparrow + 5CO_2 + 7H_2O + 6OH^-$. Requires specialized acclimation for Methyloversatilis.
  • Alternative External Sources: Micro-C (proprietary carbohydrate/glycerin blends), pure glycerin, or sodium acetate ($CH_3COONa$).

BNR System Configurations

1. Modified Ludzack-Ettinger (MLE) Process

• Configuration Architecture: A Pre-Anoxic Basin followed in series by an Aerobic Basin, followed by a Secondary Clarifier.
• Internal Mixed Liquor Recycle (IMR / Nitrate Recycle): A high-capacity, low-head axial flow submersible pump recycles large volumes of nitrified mixed liquor from the effluent end of the aerobic basin back to the pre-anoxic basin at 200% to 400% of forward influent flow ($Q$).
• Process Elegance:
  • The pre-anoxic basin utilizes the raw influent's readily biodegradable organic carbon ($BOD_5$) to drive denitrification, eliminating the cost of external carbon addition.
  • Denitrification in the pre-anoxic zone reduces the mass of BOD entering the aerobic basin, saving 15% to 25% of aeration blower energy.
  • Generates $3.57\text{ lbs}$ of alkalinity per lb $N$ reduced, buffering the downstream nitrification basin.
• Effluent Total Nitrogen Performance: Typically achieves $6\text{ to } 8\text{ mg/L}$ Total Nitrogen, comfortably complying with Arizona Class A+ reclaimed standards ($<10\text{ mg/L}$). Nitrate removal efficiency is governed by the recycle ratio:

$\text{Theoretical Removal Efficiency} = \frac{R_{IMR} + R_{RAS}}{1 + R_{IMR} + R_{RAS}}$

(At $R_{IMR} = 3.0$ and $R_{RAS} = 0.5$, theoretical removal is $3.5 / 4.5 = 77.8%$).

2. Four-Stage Bardenpho Process

• Configuration Architecture: Four sequential biological zones in series:
  1. Primary Anoxic Zone: Receives raw influent, RAS, and 400% IMR recycle; removes ~75% of nitrate using influent carbon.
  2. First Aerobic Zone: Completes carbonaceous BOD oxidation and achieves full nitrification of ammonia to nitrate.
  3. Secondary Post-Anoxic Zone: An anoxic basin receiving no raw wastewater. Denitrification relies on slow endogenous decay of biomass (or supplemental carbon dosing like Micro-C) to reduce residual nitrate.
  4. Re-aeration (Polishing) Zone: A small aerobic tank (10 to 30 minutes HRT) that strips entrained nitrogen ($N_2$) gas bubbles from flocs (preventing clarifier rising sludge), re-oxygenates mixed liquor to $4.0\text{ to } 6.0\text{ mg/L}$, and prevents secondary clarifier phosphorus release.
• Effluent Performance: Consistently produces effluent Total Nitrogen $<3.0\text{ mg/L}$.

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Biological & Chemical Phosphorus Removal

Phosphorus exists in wastewater primarily as soluble orthophosphate ($PO_4^{3-}$), polyphosphates, and organically bound phosphorus. Phosphorus removal can be achieved biologically, chemically, or through a hybrid approach.

Enhanced Biological Phosphorus Removal (EBPR) Cycle:

    ┌────────────────────────────────────────────────────────┐
    ▼                                                        │
[ ANAEROBIC SELECTOR ]                               [ AEROBIC BASIN ]
• DO = 0.0, NO3 = 0.0                                • DO > 2.0 mg/L
• PAOs consume VFAs (Acetate)                        • PAOs metabolize stored PHAs
• Store PHAs internally                              • "Luxury Uptake" of Orthophosphate
• Break Poly-P bonds ──► RELEASE PO43- INTO WATER   • Store Poly-P granules ──► PURGE VIA WAS
    │                                                        ▲
    └────────────────────────────────────────────────────────┘

1. Enhanced Biological Phosphorus Removal (EBPR)

• Microbiology: Driven by Phosphorus Accumulating Organisms (PAOs), predominantly Candidatus Accumulibacter phosphatis.
• Anaerobic Selector Mechanism (DO = 0.0 mg/L, $NO_3^- = 0.0\text{ mg/L}$):
  • Under true anaerobic conditions (absence of both oxygen and nitrate), standard heterotrophs cannot function.
  • PAOs break high-energy intracellular polyphosphate bonds to generate cellular ATP energy.
  • PAOs use this energy to absorb Volatile Fatty Acids (VFAs), such as acetic acid and propionic acid, and store them internally as polyhydroxyalkanoates (PHAs) / polyhydroxybutyrate (PHB).
  • In doing so, PAOs release orthophosphate ($PO_4^{3-}$) into the bulk solution, causing dissolved phosphorus concentrations in the anaerobic basin to spike up to $20\text{ to } 40\text{ mg/L}$.
• Aerobic Zone Mechanism (DO > 2.0 mg/L):
  • In the presence of oxygen, PAOs metabolize their stored intracellular PHAs to generate copious energy for cellular growth.
  • PAOs perform luxury uptake of soluble orthophosphate from the bulk liquid, taking up substantially more phosphate than they released in the anaerobic zone.
  • Phosphate is polymerized into intracellular polyphosphate granules, increasing cellular phosphorus content from a normal bacterial level of 1.5%–2.0% up to 4.0% to 8.0% of dry cell weight.
• Final Removal via WAS: Phosphorus is permanently removed from the treatment train by wasting phosphorus-rich biomass via Waste Activated Sludge (WAS).
• Crucial Operational Prerequisite: EBPR requires adequate influent VFAs (optimal ratio $>25:1\text{ rbCOD:P}$ or $>10:1\text{ VFA:P}$). In Arizona facilities, where hot sewers often consume sugars, on-site primary sludge fermenters are commonly used to generate VFAs.

2. Chemical Phosphorus Precipitation

When wastewater lacks sufficient VFAs for EBPR, or to polish effluent to ultra-low concentrations ($<0.1\text{ mg/L}$ Total Phosphorus), metal salts are dosed to precipitate insoluble metal phosphate complexes:

• Aluminum Sulfate (Alum, $Al_2(SO_4)_3 \cdot 14H_2O$):

$Al^{3+} + PO_4^{3-} \rightarrow AlPO_4\downarrow \text{ (Aluminum Phosphate)}$

• Ferric Chloride ($FeCl_3$):

$Fe^{3+} + PO_4^{3-} \rightarrow FePO_4\downarrow \text{ (Ferric Phosphate)}$

• Practical Dosing Stoichiometry: While the theoretical molar ratio is $1:1$ ($Al:P$ or $Fe:P$), competing side reactions with natural wastewater alkalinity (precipitating $Al(OH)_3$ or $Fe(OH)_3$) necessitate an actual operational dosing molar ratio of $1.5:1\text{ to } 2.5:1$ (equivalent to $2.0\text{ to } 3.5\text{ lbs Fe per lb P}$ removed, or $1.5\text{ to } 2.5\text{ lbs Al per lb P}$ removed).
• Operational Impacts:
  • Sludge Production: Chemical precipitation increases total dry solids sludge production by 15% to 35%.
  • Alkalinity Depletion: Dosing acidic metal salts consumes natural alkalinity, depressing pH and risking nitrification failure if unmonitored.